In an exhaust passage of an internal combustion engine, there is arranged a catalyst which serves to adsorb and remove exhaust gas components discharged from the internal combustion engine. At the time of starting up of the internal combustion engine, the catalyst is in an inactive state and the performance of adsorption and removal thereof decreases. For that reason, there has been disclosed a technique in which at the time of starting up of an internal combustion engine, gas fuel is used, and after a set period of time has elapsed from the engine starting up, the internal combustion engine is operated by changing the gas fuel to liquid fuel (for example, refer to a first patent document). According to this, in the first patent document, it is intended to provide good startability as well as to improve exhaust emissions at the time of the engine starting up. In addition, by suppressing the use of the gas fuel as much as possible, it is also intended to secure a sufficient cruising distance.